We previously reported the synthesis and pharmacological activity of a series of new compounds which directly inhibit the human blood platelet thromboxane A2/prostaglandin H2 (TXA2/PGH2) receptor. It was found that these agents, in particular 13-azaprostanoic acid (13-APA)2, specifically antagonize platelet activation induced by TXA2/PGH2, with virtually no effects on platelet activation induced by other physiological agonists. In the present proposal, studies have been designed to now employ these agents as selective pharmacological tools to investigate the underlying mechanism associated with TXA2/PGH2-induced platelet activation. Specifically, experiments are proposed to perform binding studies using (3H) 13-APA in intact platelets and platelet membrane preparations. These studies should provide the first direct evidence for the existence and location of a platelet TXA2/PGH2 receptor. In order to identify and characterize the protein composition of this presumed receptor, a radiolabeled alkylating derivative of 13-APA will be employed. In addition, experiments are proposed to more clearly establish the relative ability of TXA2 and PGH2 to cause platelet activation. Through the use of a competitive receptor binding assay developed in this proposal and selective TXA2 synthetase inhibitors, it should be possible to establish whether or not PGH2 is itself capable of interacting with a platelet receptor. Finally, we will explore the possibility that direct TXA2/PGH2 receptor antagonism alone or in combination with prostacyclin may serve as an effective means of reducing platelet reactivity in vivo. In addition to the above studies a synthetic program will be undertaken to develop new agents which should be of substantial value in further characterizing the involvement of TXA2/PGH2 in platelet activation. This program will involve the synthesis of new compounds to investigate the structural requirements for interaction with the TXA2/PGH2 receptor, as well as the synthesis of a modified 13-azaprostanoid (for affinity chromatography) to purify the receptor site. The breadth of this proposal therefore extends from the molecular interaction of TXA2 with its receptor to the application of TXA2-antagonists to the inhibition of platelet activation in vivo. This approach should provide a more rational basis for the future development of pharmacological agents useful in the treatment of thromboembolic disorders.